Many technologies which measure the surface form of an object have been proposed. They are roughly divided into two types: one type measures one point at a time and the other type measures many points simultaneously. Although the one point measurement type is highly accurate and reliable, a large amount of measurement time, from several tens of minutes to several hours, is needed when measuring a whole object surface. On the other hand, while the many point simultaneous measurement type has a feature of high-speed, it has difficulties in respect to reliability and accuracy. A light section method, grating projection phase shifting method, and measuring method using confocal microscopy (hereinafter referred to as “confocal method”), all of which are many point simultaneous measurement types, have high reliability compared with other many point simultaneous measurement methods that have been proposed at the laboratory level and have already been used practically. Although these methods are high in speed compared to the one-point measurement type methods, it cannot be said that these are sufficiently high in speed for in-line inspection in the FA field.
As will be explained later in detail, the light section method, the grating projection phase shifting method, and the confocal method need an imaging device as a detector, and some type of scanning which requires time. Usually, images are acquired with the imaging device for every partial scanning. After many repetitions of this process, the measurement is completed. In practice, one measurement (of one field of view) involves several tens to several hundreds of images either by the light section method or the confocal method. A long measurement time is unavoidable since an imaging cycle of a TV camera as an imaging device is about 30 images per second. The grating projection phase shifting method, although relatively high in speed, needs at least three images which are captured at the different time, therefore measurement of moving objects is still impossible. The light section method, the grating projection phase shifting method, and the confocal method are described below in detail.
FIG. 11 is an example of the measurement system proposed on the basis of the light section method. This system is highly reliable with many practical applications as indicated in the non-patent reference 1. The figure here shows only one slit light scanning part on a side in order to explain only the main points of the above-mentioned reference, while the slit light scanning parts are on both the right and left sides in the reference.
Images are continuously input to the image processing device 115 by a television camera 114, while light from a laser slit light source 111, scanned by a scanning mechanism 112, irradiates an object 10 from an angle different from the optical axis of the imaging lens 113. In one image as shown on the display device 116 of FIG. 11, one slit will appear distorted according to any irregularities on the surface of the object 10. While in FIG. 11 the slit light moves from the right to the left until a slit scan is completed, images of 256 sheets to 512 sheets are input. For every pixel of the image input, the image processing device 115 detects the timing (for example, tp in the figure) at which the value of the pixel becomes the largest, that is, when the slit light passes over the position on the object 10 with which the pixel corresponds, and calculates the three-dimensional position of the object 10 surface as intersection P between the projection angle of the slit light at that time and the angle of the main beam of the imaging lens 113 determined by the position of each pixel. (Uesugi Mitsuaki, 1993, The Optical Three-Dimensional Measurement edited by Toru Yoshizawa, Shin-Gijyutu Communications, page 39–52)
FIG. 12 is an example of a measurement system which uses the confocal imaging system 121. As the confocal imaging system 121, any one of a laser scanning microscope, a Nipkow board scanning microscope, a non-scanning confocal imaging system or the like can be used, and the figure is simplified since any one of these is sufficient.
The main feature of the confocal imaging system 121 is that only the position 122, which is in focus, is imaged, i.e., hardly any light from the portion which is out of focus will reach a detector 123. The feature is called optical sectioning. When the image is continuously input using the detector 123 while moving an object 10 in the optical axis direction by a Z stage 124, only the in focus portion in the field of view is imaged as shown in the display device 116 of FIG. 12 and this portion expresses the contour line. While the object being moved downward from top to bottom in the figure until a scan of the Z stage 124 is completed, about several hundreds of images are input. The image processing device 115 will detect the timing for each pixel when the value of the pixel becomes maximum. That is, the optical system focuses on the position of the object 10 with which the pixel corresponds, and the position of the Z stage 124 at that time will itself express the relative height of the surface of the object 10.
Next, the grating projection phase shifting method is briefly explained using FIG. 13. The grating projecting method projects a plurality of slit light rays simultaneously on an object while the light section method projects one slit light ray. A so-called sinusoidal grating 132 of the phase shifting method, which will be explained below, is used to make transmittance changes in a sinusoidal curve as shown in FIG. 14. The image of the sinusoidal grating 132 illuminated by a lighting source 131 is projected onto the surface of an object 10 by the projection lens 133, and is imaged by an imaging lens 113 and a television camera 114 from a different angle. If the phase of the grating pattern projected for each pixel of the obtained image is known, the relative relief of the surface of the object 10 can be obtained. The phase can be determined by the phase shifting method. By shifting the sinusoidal grating 132 by a known value at least twice with the phase shifter 134, at least three images of the projected grating with different phases are taken. More than three values will be obtained for every pixel from at least three or more images with different phases, and since these values are considered to be values sampled from the sinusoidal curve, the phase can be obtained as the initial phase by fitting to the sinusoidal curve.